Weak Characters are new Characters that are slighty weaker than other Characters and start with an have a normal maximum of three Lives. They are fun to play with if you want a real challenge.

The other thing Weak Characters are good for is lowering your Power Level. All Weak Characters have a Power Level of 0! This gives you lots of options to add all sorts of things to them.
If you don't know what a Power Level is and if you haven't ran across it yet then click here.